Formation Water Genesis Mechanism of Deep Tight Sandstone Gas Reservoirs in the Southern Ordos Basin and Its Indication for Development.

By analyzing the chemical characteristics of the formation water in the tight sandstone reservoirs of the P2x8 and P1s1 in the southern Ordos Basin, combined with rock mineral composition, reservoir physical properties, and well gas testing data, the genesis mechanism of formation water and its guiding role in gas reservoir development were discussed. The results show that the formation water is derived from the mixture of syngenetic seawater and meteoric water and has undergone remarkable modification by water-rock interactions, showing characteristics of Ca enrichment and Mg and SO depletion. The albitization of plagioclase in reservoir rock components causes Ca excess and Na deficiency in formation water, while the chloritization of albite leads to the increase of Na. The dolomitization of calcite contributes to the decrease of Mg in the formation water, while later ankerite formation makes the correlation of Ca excess-Mg deficit worse. In addition, the chloritization of dolomite and the formation of calcified debris by the metasomatism of ferrocalcite further increase the Ca content of the formation water. Fe reduction and pyrite formation in reservoir minerals reduce the concentration of SO . The study also show that reservoirs endowed with Ca-Cl type formation water has more production potential. Chlorite fill, argillaceous debris, and calcareous debris are negatively correlated with face rate, porosity, and permeability. It is innovatively proposed that the specific formation chemical characteristics combinations and can effectively characterize reservoir porosity and permeability, while combinations and are strongly correlated with gas production and water production. In conclusion, the hydrochemical characteristics of geological formations are important references for reservoir evaluation and development indicators, which is helpful to accurately evaluate the development potential of reservoirs and guide exploration strategies of gas reservoirs.